Recently, noises and vibration problems have become an objective of public concern as an environmental pollution with development of transportation means and increase in residential areas which are located near factories and the like. Further, in a workshop, there is a tendency to limit noises and vibration to improve working atmosphere. To cope with this tendency, it is requested to impart vibration damping property to a metallic material, namely, to impart a function to a noise generating member itself so that the member can absorb its own vibrational energy and convert it into heat energy to attenuate frequency of vibration or vibrational amplitude, thereby noise is decreased. Further, it is requested to improve such a function.
Based on these requests, as one of vibration damping materials having desired properties, there has been proposed a vibration damping material having a composite laminate structure wherein a middle layer having viscoelasticity is sandwiched by metal layers. This type of a composite vibration damping material has been studied and employed as oil pans of automobiles, engine covers, chutes of hoppers, stopper of conveying apparatus, domestic electric equipments, vibration reducing members of other metal processing machines, structural members of precision machines in which prevention of vibration is desirable and the like.
In general, vibration damping property of such a composite vibration damping material depends upon the properties of a viscoelasticity layer which constitutes the middle layer thereof. When vibration damping property is expressed as a loss factor (which is a measure of conversion of an external vibration energy into a heat energy by internal friction, and corresponds to a value relating to mechanical hysteresis loss due to vibration), the property shows a peak at a certain temperature. It has been known that it is most effective to use a vibration damping material at about this temperature showing the peak property.
Hitherto, as a viscoelastic composition which constitutes the middle layer of such a composite vibration damping material, there have been known a simple polyester (Japanese Patent Kokai No. 50-143880) or a polyester to which a plasticizer is added (Japanese Patent Kokai No. 51-93770); a simple polyurethane foam (Japanese Patent Kokai No. 51-91981); a simple polyamide (Japanese Patent Kokai No. 56-159160); a simple ethylene-polyvinyl acetate copolymer (Japanese Patent Kokai No. 57-34949); a composition of a polyvinyl butyral or a polyvinyl butyral and a polyvinyl acetate to which a plasticizer and a tackifier are added (Japanese Patent Kokoku No. 55-27975); a copolymer of a isocyanate prepolymer and a vinyl monomer (Japanese Patent Kokoku No. 52-26554); copolymers disclosed in Japanese Patent Kokoku Nos. 39-12451 and 45-34703 and Japanese Patent Kokai No. 62-74645; and the like.
Further, in Japanese Patent Kokai No. 63-56522, there is disclosed an amorphous polyester showing excellent vibration damping property at 80.degree. to 100.degree. C. However, it is used in a thermoplastic state and, therefore, adhesion at a high temperature (80.degree. to 100.degree. C.) is insufficient and durability is also insufficient.
As the properties which are required for a composite vibration damping material, first of all, vibration damping property must be high. This is generally expressed by the magnitude of a loss factor. Secondly, a composite vibration damping material is also used as structural members and is subjected to processing such as press and the like and, therefore, it is also required that adhesive strength between a viscoelastic middle layer composed of a viscoelastic resin and a metal layer, particularly, adhesive strength under shear must be high. Thirdly, sometimes, a pressed composite vibration damping material is further subjected to a baking finish process and it is heated to about 200.degree. C. and, therefore, it is also required that the viscoelastic middle layer does not run-off at about the above temperature. Fourthly, it is used as structural components of various structures such as machines and the like and, therefore, durability is also required.
A composite vibration damping material made of the above known viscoelastic composition has problem in any of these properties and is unsatisfactory.
Particularly, in a vibration damping material having a composite laminate structure, a temperature range showing a peak vibration damping property is at above a glass transition temperature of the viscoelastic middle layer resin. When a temperature rises to higher than the glass transition temperature, adhesion which remarkably influences on durability is rapidly lowered.
In general, it is difficult to obtain both satisfactory vibration damping property and durability in a composite laminate structure. Particularly, in the case of a material which is usually used in a high temperature range between 60.degree. to 160.degree. C. and, at the same time, vibration damping property is required at this temperature range, no material which satisfies both properties has not been found.
It has been known that an amorphous polyester resin is superior in vibration damping property (Japanese Patent Kokai Nos. 62-295949, 63-75056, 63-186757 and 63-56522). However, in the amorphous polyester resin, the above tendency is also remarkable. Particularly, when a temperature rises to higher than the glass transition temperature, durability is lowered due to decrease in adhesion, hydrolysis of the resin and the like.
In a crystalline polyester resin, durability at a temperature higher than the glass transition is insufficient and vibration damping property is inferior in comparison with the amorphous polyester resin.
Further, in order to improve durability, curing of the middle layer can be applicable. However, even if it is cured by using an isocyanate crosslinking agent which is normally used, sufficient durability can not be obtained. Further, even if an epoxy curing agent is used, a material which satisfies all of vibration damping property and adhesion at a high temperature range as well as durability can not be obtained.
As described above, no vibration damping material which satisfies various properties at a high temperature range and good durability has not been obtained according to a conventional technique.